The development of photovoltaic devices such as solar cells is important for providing a renewable source of energy and many other uses. The demand for power is ever-rising as the human population increases. In many geographic areas, solar cells may be the only way to meet the demand for power. The total energy from solar light impinging on the earth for one hour is about 4×1020 joules. It has been estimated that one hour of total solar energy is as much energy as is used worldwide for an entire year. Thus, billions of square meters of efficient solar cell devices will be needed.
Photovoltaic devices are made by a variety of processes in which layers of semiconducting material are created on a substrate. Layers of additional materials are used to protect the photovoltaic semiconductor layers and to conduct electrical energy out of the device. Thus, the usefulness of an optoelectronic or solar cell product is in general limited by the nature and quality of the photovoltaic layers.
One way to produce a solar cell product involves depositing a thin, light-absorbing, solid layer of the material copper indium gallium diselenide, known as “CIGS,” on a substrate. A solar cell having a thin film CIGS layer can provide low to moderate efficiency for conversion of sunlight to electricity.
Making a CIGS semiconductor generally requires using several source compounds and/or elements which contain the atoms needed for CIGS. The source compounds and/or elements must be formed or deposited in a thin, uniform layer on a substrate. For example, deposition of the CIGS sources can be done as a co-deposition, or as a multistep deposition. The difficulties with these approaches include lack of uniformity, purity and homogeneity of the CIGS layers, leading ultimately to limited light conversion efficiency.
For example, some methods for solar cells are disclosed in U.S. Pat. Nos. 5,441,897, 5,976,614, 6,518,086, 5,436,204, 5,981,868, 7,179,677, 7,259,322, U.S. Patent Publication No. 2009/0280598, and PCT International Application Publication Nos. WO2008057119 and WO2008063190.
Other disadvantages in the production of thin film devices are limited ability to control product properties through process parameters and low yields for commercial processes. Absorber layers suffer from the appearance of different solid phases, as well as imperfections in crystalline particles and the quantity of voids, cracks, and other defects in the layers. In general, CIGS materials are complex, having many possible solid phases. Moreover, methods for large scale manufacturing of CIGS and related thin film solar cells can be difficult because of the chemical processes involved. In general, large scale processes for solar cells are unpredictable because of the difficulty in controlling numerous chemical and physical parameters involved in forming an absorber layer of suitable quality on a substrate, as well as forming the other components of an efficient solar cell assembly, both reproducibly and in high yield.
For example, introducing alkali ions at a controlled concentration into various layers and compositions of a CIGS-based solar cell has not been achieved in a general way. Conventional methods for introducing sodium do not readily provide homogenous concentration levels or control over sodium location in a CIGS film. The presence and level of alkali ions in various layers is a chemical parameter that should be controlled in a solar cell manufacturing process.
What is needed are compounds, compositions and processes to produce materials for photovoltaic layers, especially thin film layers for solar cell devices and other products.